Add a relay regression test for the join ordering used by gateway MAC-state
seeding. The test sends a second client's hello but intentionally delays reading
that client's welcome until after the existing peer receives PeerJoined.
This guards the ordering from the relay admission path: existing peers are
notified before the joining peer can proceed from its welcome and begin sending
Ethernet datagrams. That matters for first DHCP/ARP frames after a Windows
client joins a room with an existing LAN gateway.
Test Plan:
- cargo test -p lanparty-relay notifies_existing_peer_before_join_welcome
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- git diff --cached --check
Refs: PLAN.md MVP relay lifecycle and gateway MAC learning
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